Processes for the selective epoxidation of olefins which contain no allylic hydrogen atoms (non-allylic olefins) or olefins which contain hindered allylic hydrogen atoms are described by Monnier and Muehlbauer in U.S. Pat. Nos. 4,897,498, 4,950,773, 5,081,096, 5,138,077 and 5,145,968. Stavinoha and Tolleson disclose in U.S. Pat. No. 5,117,012 the selective epoxidation of 1,3-butadiene (butadiene) to 3,4-epoxy-1-butene (EpB) by contacting a mixture comprising 1,3-butadiene, oxygen and methane with a supported silver catalyst at elevated temperatures. Stavinoha, Monnier, Hitch, Nolen and Oltean describe in U.S. Pat. No. 5,362,890 the advantages resulting from the use of a C.sub.2 -C.sub.6 paraffin hydrocarbon as an inert diluent in the feed gas in the epoxidation of certain olefins such as 1,3-butadiene.
The epoxidation of olefins such as butadiene in the presence of a modified, supported, silver catalyst to an epoxide such as 3,4-epoxy-1-butene according to known processes such as those described in the above referenced patents results in the co-production of a high-boiling, organic, resinous material. Generally, the rate of formation of the foulant material is dependent, at least in part, upon pressure within the epoxidation reactor with increased pressures resulting in increased rates of foulant formation. This foulant material coats the catalyst and thereby decreases catalyst activity which, in turn, results in lower conversion of the olefin reactant and production of the desired epoxide. This foulant also may coat and foul process equipment, thereby impeding or obstructing gas flow through the reactor and associated equipment. Build-up of the foulant material over extended periods of operation of the continuous epoxidation process significantly shortens the length of service of catalyst, results in an unacceptable pressure drop through the reactor during normal operation, lowers catalytic activity, and ultimately results in complete blockage of gas flow through the reactor. Removal of catalyst material coated or covered with the organic foulant from the reactor tubes typically used in the epoxidation of olefins is extremely difficult, requiring considerable time to remove the foulant from the reactor tubes and associated equipment downstream from the tubular reactor.
U.S. Pat. Nos. 5,618,954 and 5,905,161 are directed to reducing foulant formation during the epoxidation of butadiene. These patents disclose the use of 5-80% water in the feed gas to the epoxidation reactor to lower the rate of foulant formation. However, even with the water addition, fouling occurs and the supported silver catalyst requires frequent regeneration in a gas stream containing oxygen and water vapor. Furthermore, the addition of water to a process gas stream containing reactive epoxides can result in the formation of the corresponding diols, e.g., 3-butene-1,2-diol and 2-butene-1,4-diol, which are reactive species that can further react to form organic foulant residue. Although it is not known exactly how the organic foulant forms, it has been experimentally determined that 3,4-epoxy-1-butene, oxygen, and water are required for growth, or propagation of the organic foulant. Therefore, addition of water to suppress the formation of organic foulant is actually detrimental to the goal of lowering the rate of organic foulant formation. Finally, addition of high concentrations of water vapor described in the cited patents can result in the degradation of the modified silver catalysts that are employed to catalyze the formation of the olefin epoxides.
Inui and Tanabe, Journal of Catalysis, 52, 375-384(1978) discuss the effects of the addition of various levels of ammonia and substituted amines, such as monomethylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, and ethanolamine, etc. to the reactor feedstream during ethylene epoxidation. The authors report that the addition of ammonia and such amines in the range of 200-34000 ppmv to the reactor feedstream resulted in decreases in the rate of ethylene oxide formation. At the higher levels, catalytic activity was completely suppressed.